(1) Field of the Invention
The present invention relates to piezoelectric/electrostrictive film type elements of a unimorph type or a bimorph to be used as various transducers and actuators, etc. In particular, the invention is aimed at advantageously improving durability of such piezoelectric/electrostrictive film type elements without deteriorating their piezoelectric/electrostrictive performance by controlling a heterophase-occurrence rate at a surface of a piezoelectric/electrostrictive layer.
The piezoelectric/electrostrictive film type elements according to the present invention are intended to include elements for converting electric energies to mechanical energies, i.e., mechanical displacements, stress or vibration as well as elements for effecting reverse conversions thereof. Since the elements according to the present invention have a dielectric property as well as the piezoelectric/electrostrictive performance, they can be also used as filmy condenser elements, etc.
(2) Related Art Statement
The piezoelectric/electrostrictive elements are used in a wide variety of fields including the following: various transducers for converting electric energies to mechanical energies, i.e., mechanical displacements, forces or vibration, as well as elements for effecting reverse conversions thereof, various actuators; functional parts operating at a predetermined frequency range such as filters; various display devices such as displays; sound-emitting members such as loudspeakers; sensors such as microphones; and ultrasonic wave sensors, etc.
For example, FIG. 1(a) illustrates a known piezoelectric/electrostrictive element which includes a ceramic substrate 1 functioning as a vibrating plate and a film-type piezoelectric/electrostrictive operating section 5 provided on the substrate 1. The piezoelectric/electrostrictive operating section is constituted by a lower, first electrode film 2, a piezoelectric/electrostrictive layer 3 and an upper, second electrode film 4, as discussed in JP-A-3-128,681. In addition, there is also known a piezoelectric/electrostrictive element, which is shown in FIG. 1(b), in which a ceramic substrate 1 is provided with a cavity having a bottom portion used as a vibrating section 1a. A piezoelectric/electrostrictive operating section 5 is integrally formed on the outer surface of the vibrating section 1a as discussed in JP-A-5-49,270.
As ceramic substrates constituting such piezoelectric/electrostrictive elements, ceramic materials composed mainly of zirconium oxide partially stabilized with yttrium oxide are generally known (for example, JP-A 5-29,675, JP-A-5-97,437 and JP-A-5-270,912).
Environments in which piezoelectric/electrostrictive devices are used have recently and variously changed with diversification in the use of piezoelectric/electrostrictive devices. Particularly, when the use environment in which the above-mentioned piezoelectric/electrostrictive film type elements are used is of a higher temperature and more humid atmosphere compared with conventional conditions, deterioration in a material used for the substrate poses a problem.
That is, if the piezoelectric/electrostrictive film element is used in the above higher temperature and more humid environment, there is a tendency that the substrate begins to be degraded in the piezoelectric/electrostrictive film type element.
The present invention has been developed in view of the above actual circumstances, and is aimed at the provision of integrated piezoelectric/electrostrictive film type elements having excellent durability and an advantageous producing process thereof, without causing degradation in material of the substrate or lowering the piezoelectric/electrostrictive performance or deteriorating the characteristics of the piezoelectric/electrostrictive layer, even if used under the high-temperature and highly humid atmosphere.
In the following, the elucidation process of the present invention will be explained.
After having repeatedly performed strenuous investigations to accomplish the above object, the present inventors discovered that the substrate was deteriorated by a lead element entering into the substrate during the production process. That is, although a ceramic material composed mainly of zirconium oxide has been formerly used as the substrate, it was clarified that the lead element entering into such a ceramic substrate rapidly damaged the durability of the substrate when used in the high-temperature and highly humid condition.
Therefore, the present inventor then examined causes why the lead element entered into the substrate, and discovered that such a lead element entered the substrate when firing (thermally treating) the piezoelectric/electrostrictive material. That is, a lead-containing materials having excellent properties (such as lead zirconate titanate, etc.) was generally used as the piezoelectric/electrostrictive material, and a film of such a material was ordinarily formed as a piezoelectric/electrostrictive layer, and integrated by firing.
When a piezoelectric/electrostrictive layer made of such a lead-containing material is to be formed (by firing), the atmosphere is controlled under a condition with a high lead concentration to prevent the evaporation of the lead element contained in the piezoelectric/electrostrictive material during firing, since such evaporation may cause changes in the composition of the piezoelectric/electrostrictive layer to thereby deteriorate the piezoelectric/electrostrictive performance.
As mentioned above, the piezoelectric/electrostrictive layer is fired with a lead concentration-increased atmosphere in the presence of an evaporating source of a lead-containing material or the like so as to prevent the deterioration in the above characteristics. However, when the piezoelectric/electrostrictive layer is fired in the atmosphere with such a high lead concentration, the lead element in the atmosphere penetrates into the ceramic substrate composed mainly of zirconium oxide, which causes deterioration in the quality of the substrate as mentioned before.
In order to solve the above problems, the present inventors repeatedly performed numerous experiments and examinations, and acquired the following knowledge:
(1) The firing atmosphere need not always be an atmosphere having a high concentration of lead, even if the piezoelectric/electrostrictive layer contains the lead element. If some lead element evaporates from the piezoelectric/electrostrictive layer during firing, no deterioration occurs in the piezoelectric/electrostrictive layer;
(2) The rate of a heterophase occurring at a surface of the piezoelectric/electrostrictive layer during firing is preferable as an index for the judgment of the evaporated amount of the lead element from the piezoelectric/electrostrictive layer; and
(3) The integrated piezoelectric/electrostrictive film type element in which the area rate of the heterophase occurring during firing is controlled to within a range of 0.1 to 30% by adjusting the firing conditions suffers from neither degradation in quality in the substrate and deterioration in the characteristics of the piezoelectric/electrostrictive layer even when in use under a high-temperature and highly humid atmosphere.
The present invention is based on the above recognition. That is, the substantial features of the present invention are as follows.
An integrated piezoelectric/electrostrictive film type element having excellent durability includes a substrate made of a ceramic material composed mainly of completely stabilized or partially stabilized zirconium oxide, and a piezoelectric/electrostrictive operating section integrated onto the ceramic substrate using a film-forming method. The piezoelectric/electrostrictive operating section includes a lower electrode, a piezoelectric/electrostrictive layer of a lead element-containing composition, and an upper electrode. A heterophase-occurrence rate at a surface of the piezoelectric/electrostrictive layer is controlled to be within a range of 0.1 to 30%.
The integrated piezoelectric/electrostrictive film type element discussed above includes a ceramic substrate shaped in the form of a thin diaphragm portion, with the piezoelectric/electrostrictive operating section integrally formed on an outer surface of the diaphragm portion.
The integrated piezoelectric/electrostrictive film type element of the present invention includes a heterophase-occurence rate controlled to within a range of 1 to 10%.
The integrated piezoelectric/electrostrictive film type element of the present invention includes average grain sizes of crystals constituting the ceramic substrate with a range of 0.1 to 2.0 xcexcm.
The integrated piezoelectric/electrostrictive film type element of the invention has a thickness of the piezoelectric/electrostrictive layer of not more than 100 xcexcm. A thickness of the piezoelectric/electrostrictive operating section is not more than 150 xcexcm. A thickness of the diaphragm portion is not more than 50 xcexcm.
A process for producing an integrated piezoelectric/electrostrictive film type element having excellent durability, includes the steps of preparing a substrate made of a ceramic material composed mainly of completely stabilized or partially stabilized zirconium oxide, and successively forming a lower electrode, a piezoelectric/electrostrictive layer of a lead element-containing composition, and an upper electrode on the ceramic substrate by a film-forming method. The lower electrode, piezoelectric/electrostrictive layer and the upper electrode constituting a piezoelectric/electrostrictive operating section and at least the piezoelectric/electrostrictive layer is fired. A produced rate of a heterophase occurring at a surface of the piezoelectric/electrostrictive layer is controlled to be within a range of 0.1 to 30% in terms of an area rate by adjusting a concentration of lead in a firing atmosphere and/or an amount and a speed of a running fluid of the firing atmosphere.
In the highly durable, integrated piezoelectric/electrostrictive film type element-producing process discussed above, the atmosphere is controlled by adjusting at least one condition selected from the following four conditions: i) a composition ratio, a configuration, a weight and an arranged location of an evaporating source containing lead as a constituent element; ii) an arranged location of a piezoelectric/electrostrictive material in a firing furnace or a firing vessel; iii) an opening degree of the firing furnace or the firing vessel; and iv) feeding an absorbent for absorbing a lead element in the firing atmosphere.
These and other objects, features and advantages of the invention will be appreciated upon reading the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes could be made by the skilled person in the art to which the invention pertains.